Staphylococcus aureus is a Gram-positive bacterium that causes a variety of suppurative (pus-forming) infections and toxinoses in humans. It causes superficial skin lesions (such as boils, styes and furunculosis), more serious infections (such as pneumonia, mastitis, phlebitis, meningitis, and urinary tract infections), and deep-seated infections (such as osteomyelitis and endocarditis). S. aureus is a major cause of hospital acquired (nosocomial) infections of surgical wounds and infections associated with indwelling medical devices. S. aureus causes food poisoning by releasing enterotoxins into food, and toxic shock syndrome by releasing superantigens into the blood stream.
S. aureus expresses a number of factors that interfere with host defense mechanisms. One such factor is Protein A. Protein A is a surface protein of S. aureus which binds the Fc region of immunoglobulins. In serum, the bacteria will bind IgG molecules in an orientation on their surface which disrupts opsonization and phagocytosis. Mutants of S. aureus lacking protein A are more efficiently phagocytosed in vitro, and mutants in infection models have diminished virulence. Protein A binds with high affinity to human IgG1, IgG2, and IgG4 as well as mouse IgG2a, IgG2b, and IgG3. Protein A binds with moderate affinity to human IgM, IgA and IgE. It does not bind with human IgG3 or IgD, nor will it bind to mouse IgM, IgA or IgE.